AbstractA model system, utilizing shear‐sensitive protozoa, has been developed for characterizing the disruptive forces in agitated systems. The model system gives a measure of the maximum shear stresses in the apparatus being tested, and is particularly useful when tissue fragility is a factor in fermentor design. The time dependency of protozoan disruption is shown and discussed. Breakdown data in conventional stirred vessels and a laminar shear device are presented and discusse